Portable electronic devices are becoming more and more ubiquitous. Batteries typically power these portable electronic devices. Typically, rechargeable batteries are used to power these portable electronic devices. Charging the rechargeable batteries is an important consideration. The users of portable electronic devices desire products that can operate on a single charge for as long as possible and can recharge in as short a time as possible. Shortening the amount of time that it takes to recharge a battery is an important design consideration.
A known method of charging a rechargeable battery will now be discussed with reference to FIG. 10. FIG. 10 shows a graph 1000 of voltage and current versus time. Voltage is plotted along a first y-axis 1005 on the left side of the graph 1000 and current is plotted along a second y-axis 1007 on the right side of the graph 1000. Time is plotted along the x-axis 1008.
The graph 1000 shows charging voltage 1012 and charging current 1015. During a first charging time 1018 which is indicated on the graph 1000 as Δtpa1 a constant current is applied to the charging terminals. After the rechargeable battery reaches a predetermined voltage 1003 the rechargeable battery is charged using a constant voltage, shown on the graph 1000 as part of charging voltage 1012. The constant voltage is applied to the charging terminals during a second charging time 1021, which is indicated on the graph 1000 as Δtpa2. At time tpa3 1029 the charging current has reached a predetermined level and the rechargeable battery is considered charged. When the rechargeable battery is considered charged the charging current 1015 is turned off 1023. When the charging current 1015 is turned off 1023 the rechargeable battery voltage may drop 1026.
Determining when the charging current 1015 has reached the predetermined level adds complexity and expense to the battery charging circuit. While it is true that current can be calculated by measuring the voltage across a resistor, this method has several drawbacks. Adding a resistor increases energy losses in the battery charging circuit. It is less complicated to measure the voltage at a single point relative to a reference point. Another known method that uses a voltage measurement will be discussed with respect to FIG. 11.
Another graph 1100 of voltage and current versus time is shown on FIG. 11, which relates to a known method for battery charging. To overcome the difficulties in measuring current as discussed with respect to FIG. 10 voltage 1112 is used to indicate the level of charge of the rechargeable battery. However, the voltage across the terminals of the rechargeable battery is not an accurate indication of the level of charge while the charging voltage is applied to the terminals of the rechargeable battery. The current 1115 is turned off at various times during the charging of the rechargeable battery. Charging current and voltage are reapplied to the rechargeable battery after the voltage 1112 drops to a predetermined level 1118.
As is shown in FIG. 11 a disadvantage of this method of charging is that as the rechargeable battery is charged it takes longer and longer for the voltage to drop to the predetermined level 1118. The off time is shown as Δtpa4 1121, Δt pa5 1123, and Δtpa6 1126. As shown each successive off time increases in length and continues off the graph 1100. Since the rechargeable battery is not charging during these off times, this increases the amount of time it takes to charge the rechargeable battery. It will take the longest amount of time to add charge to the rechargeable battery when charging is nearly complete. A way is needed to determine when a rechargeable battery is fully charged without relying on current measurements and without turning off the charging current for long periods of time.
The method of charging batteries discussed with respect to FIG. 11 is known as pulse charging. As discussed above, when pulse charging is used to charge a rechargeable battery the amount of time that the charging current is off increases as the rechargeable battery gets closer to being fully charged. An example of a component that uses pulse charging techniques to charge a rechargeable battery is the National Semiconductor LM3626 Lithium Ion (Li—Ion) Battery Pulse Charge Controller. The LM3626 is described in National Semiconductor, LM3626 Li-Ion Battery Pulsed Charger Controller, Jan. 7, 2000, Preliminary Information.